Indeed it appears to be 
Did you download / import a new version of TMP?
Before importing the new release, did you remove the previous package first by deleting the “TextMesh Pro” folder as per the Release and Upgrade notes?
TMP includes GUISkins which in previous versions were located in “TextMesh Pro/GUISkins/…”. In the latest release these have been moved in the “TextMesh Pro/Editor Resources/…” folder. See if those are in there?